جستجوی مقالات مرتبط با کلیدواژه "stiffness" در نشریات گروه "عمران"

In general, despite their seeming insignificance, there are some details in the ground that have significant effects on soil-foundation system behavior such as slip surfaces, shear bands, and thin layers (Valor et al. 2017). Terzaghi (1929) termed “these features minor geologic details and pointed out their enormous potential effects on the safety of dams”. In the literature review very little study has been performed on the effects of a thin layer (Valor et al. 2017, Ziccarelli et al. 2017, Oda and Win, 1990)In the present paper, the influences of the horizontal thin layer on the ultimate bearing capacity of the circular foundation resting on the sandy bed were studied by implicating a small-scale physical model for the soil-foundation system. The problem of the soil- circular footing system is schematically illustrated in Fig. 1. The problem is investigated under the axisymmetric condition, and the circular foundation is rigid. This foundation rests on the ground surface, on the other hand, the initial depth of embedment is nil. The studies were performed by the material type, thickness, and depth of the thin layer variation. For the bed sand, crushed uniform silica sand (SP) with medium density was used. For the thin layer, materials with different strength properties (strong and weak) in comparison with the sandy bed were used.For the weak layer, the clay powder with CL classification was used. Clay with a natural moisture content of 5.5% and a very low density of 12.1 kN/m3 was used consistently in all of the experiments.For the strong layer, a fine-grained asphalt mixture with an unconfined compressive strength of 1460 kPa and unit weight19.12 kN/m3 was used.

Staggered truss system is one of the types of lateral bracing systems in steel structures. In this system, lateral forces are transmitted through the rigid floor diaphragm between adjacent trusses. The columns of the staggered truss system are placed only in the outer walls and the middle columns are removed. Hence, a large open space without columns is achievable, which is architecturally appropriate. In this paper, the effect of geometric characteristics of staggered truss systems such as the ratio of the opening length, the type and arrangement of the side trusses in seismic performance is investigated using ETABS software. For this purpose, the ability of the analytical model was first validated to estimate the available experimental results. It was observed that the analytical result is in good agreement with the results of the Experimental model. Then, by examining several different models, the effect of the length of the middle opening was investigated on the seismic parameters of the Staggered truss system. The results showed that the ratio of the length of the middle opening is very important in seismic performance; So that this ratio can have a direct effect on the ductility and stiffness of the structure. Finally, the effect of truss type used in side openings was evaluated on the best span ratio based on the best level of seismic performance. In this paper, three types of structures are used: short, medium and high.

In this paper, a finite element analysis of the behavior of embedded CFST columns to the foundation under axial-lateral combined loading. First, the proposed finite element model is compared and analyzed by the experimental results of previous research, which showed that the local damage, failure patterns and hysteresis curves were consistent. A detailed parametric study to evaluate the cyclic behavior of embedded CFST columns to the foundation with the characteristics, the ration diameter to thickness, embedded length, compressive strength and connection conditions CFST column to the foundation. Based on the parametric study, the values of stiffness, strength, ductility and energy for the studied specimen have been calculated. The results showed that using the proposed finite element model, weak cyclic behavior for CFST connection to the foundation in the conditions of connection with the base plate and better cyclic behavior for the CFST column and its connection to the foundation in the embedded conditions is obtained. In addition, the hysteresis behavior of the CFST column connection with the embedded stiffener plate is much better than the embedded connection without the stiffener. The rings stiffener of the hysteresis diagram changes compact condition with increasing concrete strength. In addition, lateral strength, lateral stiffness, ductility performance and cumulative energy dissipation also increased in compact specimens with increasing confine concrete. The failure modes of the CFST connection to the foundation with the base plate are the same as the embedded connection mode without the stiffener and with the ring stiffener. The failure modes in these three modes are from the connection as a tearing steel pipe at the end of the column. In the case of an embedded connection with a longitudinal stiffener, it is a diagonal crack of the concrete on the foundation. Lateral strength, lateral stiffness, ductility performance, and cumulative energy also increase with increasing steel pipe thickness. CFST column burial conditions with hardeners have a positive effect on the hysteresis rings of CFST connection to the foundation, and this type of connection has been able to significantly improve lateral strength, lateral stiffness, ductility and dissipative cumulative dissipation energy.

Shallow foundations such as strip foundations are widely used in transmitting loads from the superstructure to the supporting soils. In many cases, the ground materials are not uniform and may have thin layers, which are not usually detected in geotechnical site investigations. In this research, the effects of a thin layer on the ultimate bearing capacity of a strip foundation on the sand bed are investigated by small-scale physical models. Due to very limited research that has been carried out on the thin layer effect on the ultimate bearing capacity, it seems that further studies can understand the effect of this layer. The investigations were carried out by varying the material type, thickness, and depth of the thin layer. The results indicate that the weak thin layer decreases both the ultimate bearing capacity and stiffness of the soil-foundation system and the strong thin layer increases both the ultimate bearing capacity and the soil-foundation system stiffness. The amount of this effect depends on the thickness, depth of deposition, and material type of the thin layer. According to the results, the weak layer for the critical depth of 1.2B led to the most reduction in ultimate bearing capacity by 40%, while no effects were observed at the depth of 3.6B. The strong layer is also for the state where this layer is just below the footing, had the highest increase in ultimate bearing capacity by 76%, but at a depth of about 2.4B, it was ineffective.

The energy absorber device improves the seismic behavior of structures against seismic loads. Despite the proper performance of the dampers against seismic loads, they impose additional costs on the structure. Therefore, in this paper, a new damper is introduced which, in addition to improving the seismic behavior, imposes a small cost on the structure. To evaluate the performance of the proposed damper, 15 numerical samples were simulated by Abacus software. under cyclic loading. The study of cyclic behavior was performed on a steel frame of one story-one bay and the sensitivity of cyclic behavior was studied based on the parameters of thickness, length and dimension to the thickness of the brace and slenderness of brace. The thicknesses of the damper are, respectively, 12, 21 and 30 mm, the length of the damper is 400, 500 and 600 mm, the geometry of the damper is arc and the thickness of the brace is 12, 21 and 30 mm. Numerical simulation was performed. The results of this study showed that this type of damper has a good performance in energy dissipation imposed on the structure. Although the proposed damper improves the energy absorption of the structure, it will also reduce the stiffness.

In this research, consider to the importance of the effect of concrete strength on dynamic behavior of concrete dams, the effect of body concrete stiffness on seismic performance of roller compacted concrete (RCC) dams is investigated using Monte Carlo simulation which is the effective method for evaluation of different parameters on dynamic responses. For implementation of concrete stiffness, the modus of elasticity is considered as input parameter and its effect on output parameters including maximum response of dam crest displacement, 1st principle stress at heel, 3rd principle stress at toe and hydrodynamic pressure at heel of dam selected as critical responses is investigated. Ansys software, based on finite element method, was selected for dam modeling and seismic analysis and Nemark method has been used to solve the dynamic equation. Watana dam has been selected as case study and has been modeled in the 2-dimensional form and dam-reservoir-foundation interaction considered in model. Finally, to show the effect of modulus of elasticity of dam concrete, the responses were obtained as sensitivity curves. Obtained curves show the trend of responses to variation of modulus of elasticity obviously. According to results, one can select the optimum value of modulus of elasticity of concrete and concluded about the safety of dam consider to design criteria.

In the piled raft foundation, in contrast to the pile group, both raft and piles transfer the imposed load to the foundation soil. The concentration of shear stresses and bending moments at the connection point of the pile and raft in the piled raft may cause a structural collapse in the pile while the geotechnical bearing capacity of the pile has not fully mobilized. This problem may be solved by disconnecting the piles from raft and inserting a soil layer between the piles and the raft. This layer in non-connected piled rafts is called cushion. In a non-connected piled raft, the cushion plays an important role to mobilize the bearing capacity of the foundation soil, adjusting the load transfer mechanism, and changing the system stiffness. The behavior of a connected and non-connected piled raft is too complicated to easily estimate the load sharing ratio and stiffness for the preliminary design. In the present research, based on the test results of the pile group and the unpiled raft, an analytical approach is introduced to calculate the load sharing ratio and the stiffness of the connected and non-connected piled rafts. To verify the proposed analytical model accuracy, 21 small scale tests on the unpiled raft, pile group, connected and non-connected piled rafts were conducted. According to the results increasing the number and length of piles, increases the bearing capacity. In a non-connected piled raft, increasing the cushion thickness decreases the load sharing ratio of piles, stiffness, and bearing capacity of the system.

By development of science and technology, new structural systems have been developed and researched. In the last four decades, extensive studies have been done on the application of the steel plate shear wall systems. Steel plate shear walls are made and executed in various shapes. Trapezoidally corrugated steel plate shear wall is a relatively new structural system. In this paper, the effects of position, geometric shape, percentage, distance from the boundary elements and the use of the stiffener around the opening were investigated in the seismic parameters of the trapezoidally corrugated steel plate shear wall system including stiffness, strength, ductility and dissipation energy by using 46 models. The openings were geometrically square, vertical and horizontal rectangular, equal to 10, 14, and 18 percent area of the steel panel. This research was performed analytically using ABAQUS finite element software. The results of the study show that the square-shaped opening has better performance than the rectangular opening in all positions. Also, models with central opening have lower strength, initial stiffness and energy absorption rather than other models. Overall, the best position to create the opening in the corrugated plate is in the middle top of the plate and at the distance of 160 mm from the beam and also in the upper corner of the plate but without distance from the boundary elements. Another result of this research is that by using stiffener around the opening, initial stiffness and strength and energy absorption of the corrugated steel shear wall can increase until 41% and 25% and 24%, respectively. In addition among the different stiffeners, the plate with 60 mm width and 3 mm thickness has the best dimension for the stiffener around the opening.

Increasing the resistance of members or adding constraints to the structure does not jeopardize the overall safety of the structure. This can be proved under certain static loads by the safe theorem which is one of the fundamental theories of plastic analysis. Although this theorem has not proved in the condition of dynamic loads, its results are used in the everyday design under dynamic loads, so this paper attempts to numerically study these results under dynamic loads. Since the concept of mechanism and collapse cannot be investigated under transient loads, the level of ductility demand of structural components is considered to ensure the safety of the structure. For this purpose, the plastic rotation of the members was extracted after a minor variation in resistance and stiffness of the beams in a 2D-five-story steel moment frame by performing dynamic analysis. To compare ductility demand in dynamic analysis with criteria values, the evaluation was surveyed under nonlinear static analysis. The results analysis shows that with the increasing resistance, in most cases the ductility demand decreases, and in some cases the maximum increases by 7.3%. With increasing stiffness, the ductility demand has increased by a maximum of 16%.By comparing the results of dynamic and static analysis, it can be concluded that the required ductility in the dynamic analysis by increasing characteristics has been lower than the corresponding value in static analysis. Therefore, the increase in the partial stiffness and resistance of the beams under dynamic loads has not caused the structural collapse.

Accurate determination of the collapse moment of structures by nonlinear analyses is one of the major challenges for engineers in the seismic design of buildings. Although, structural damage can be assessed at various levels, the collapse of buildings is one of the worst events in the construction industry where casualties reach their maximum. In this research, 3D steel moment-resisting frame structures with 4, 8 and 12 story with special ductility have been subjected to nonlinear analysis including nonlinear static analysis, incremental nonlinear dynamic analysis and finally to investigate their collapse capacity, the fragility curves were used and earthquakes were considered according to FEMA P695 instruction including a pair of 22 far fault records, 14 near fault records with pulse and 14 near fault records without pulse. The models are 3D structures designed in ETABS 2016 software. The design of the structures and their seismic criteria control are based on fully validated according to standard 2800 Fourth Edition. Nonlinear structural models are also created in 3D state in OpenSees2.5.0 software. The effect of stiffness and strength deterioration is considered based on the results of the experimental models and the collapse capacity of the three-dimensional structures of the special steel moment-resisting frame is investigated probabilistically. The results show that the collapse capacity of 4, 8 and 12-story structures is the highest under far fault earthquakes and the lowest under near fault earthquakes without pulse and among the low-rise structures, The 4-story has less collapse capacity. For example, in the 4-story structure, the structural collapse capacity at statistical level 84% under near fault with and without pulse and far fault ground motions is 3.21 g, 3.61 g and 4.14 g, respectively.

Rotational friction dampers are a specific type of friction dampers which have several advantages. Dampers are used to improve the cyclic behavior of structures against forces caused by wind and earthquake. These types of dampers will cause energy dissipation by its rotating and rerotating. However, complete and comprehensive researches have not been performed on the effect of rotational friction dampers and their effect on the bearing capacity of steel frames. In this research, the behavior of concrete-filled steel tube (CFT)  in two cases frame braced with rotational friction dampers and frame braced without rotational friction dampers is investigated. For verification, the results obtained from finite element method software, ABAQUS, were compared with that of experimental studies for test samples used in a building with a height of 300m in Osaka, Japan. The hysteresis curves of the modeled samples are in good agreement with the experimental results. In order to investigate the performance of steel composite frame (with CFTs) braced with rotational friction dampers towards to steel composite frame (with CFTs) braced without rotational friction dampers  under the effect of three earthquake Far-field records, the structure was modeled, designed and analyzed in ETABS software. The use of bracing with rotational friction dampers has caused a decrease in the displacement of the roof’s center of mass for each record mentioned above which modeled in ETABS software. It decreased by 13 to 49 % for 9 records and increased by 2 to 17 % for 2 records. The use of bracing along with rotational friction damper modeled in ABAQUS software under the effect of each record has caused a decrease in base shear. The extent of these reductions was different for each record mentioned above. In each record modeled in ETABS software, the base shear of the structure has not reduced similarly; however, in some cases, the base shear has increased. It had a decrease of 11 to 37% for 7 records and an increase of 3 to 26% for 4 records.  Then a Single-storey frame with single-span With the same materials and specifications introduced in ETABS software in ABAQUS software Has been modeled. For lateral loading of columns, the lateral loading protocol based on ATC-24 and the instructions for using dampers in the design and reinforcement of buildings have been used.  According to Regulation No. 766 of the Program and Budget Organization, the loading cycles introduced in ETABS software with a frequency of 1.15T have been used in the ABAQUS Limited Components Software to move. The use of rotary braces and crankshafts in the ABAQUS limited component software under the influence of each of the discussed records has reduced the displacement of the structure relative to the structure without braces and without rotational friction dampers of the structure mentioned above was exerted under the record effect of the same earthquake in ABAQUS software The use of bracing along with rotational friction damper modeled in ABAQUS software under the effect of each record has caused a decrease in base shear. The amount of energy reduction for records understudy was not equal and varied from 8% to 34.7%. The hysteresis curves of base shear of braced structures with and without dampers are well presented.

According to the basic theorems of plastic analysis of structures, increasing the strength or stiffness of a part of the structure does not weaken it under a specific static load. This result is widely used to simplify the modeling and design of structures, but these theorems have not been proven under dynamic loading like earthquake excitations. This study applies the results of the safe theorem numerically in a two-dimensional steel moment-frame structure with five stories under nonlinear dynamic analysis with 29 different earthquake ground motion records. Under transient dynamic loading conditions, because the mechanism or collapse does not occur in the structure, the safety of the structure is investigated by maximum rotational deformation of the members. By changing the characteristics such as local stiffness and strength of the members in the range of 0.8 to 1.5 times the initial value, the maximum deformation demand of the members has been compared. The results of the dynamic analysis show that upon increasing strength, in most cases, the demand for ductility decreases and with increasing stiffness, in almost all cases, the ductility demand increases. The results of nonlinear static analysis are compared with the nonlinear dynamic analysis in cases where increasing stiffness has increased the ductility demand. From this comparison, it can be concluded that upon increasing the stiffness of the members locally, the demand for ductility in the dynamic analysis is less than its corresponding value in static analysis. As a result, it can be said that by increasing the stiffness of the structure, observing the limitations of the codes for the ductility capacity of members and connections leads to the safety of the structure. Generally, it is concluded that the safety of the structure is not compromised by minor increase in the strength and minor increase in stiffness of some members of the structure under dynamic loading if regular limitations of the codes for the ductility capacity have been observed.

Suction caissons have been used increasingly by the offshore industry throughout the world to anchor floating production systems, offshore wind turbines, oil and gas industry and anchorage elements. Suction caissons as the foundation of marine structures are usually exposed to the lateral cyclic forces of wind, waves and the impact of ships. Recent studies have considered the response of suction caissons to such loading in single-layer soil, but have generally been limited to a few thousand cycles, whereas offshore structures will generally experience millions of loading cycles over their service life. seabed soils can also contain different layers. In this article, after reviewing the results of research on lateral loading of suction caissons with a limited number of cycles, the results of recent studies with a very large number of loading cycles and about 1 million cycles are presented. Also, the long-term behavior of suction caissons to lateral cyclic loading in single-layer and layered seabed are reviewed. based on the summary of the results of existing studies, it seems that lateral cyclic loading can even in some cases increase the caisson capacity by up to 30% and stiffness by up to 50%. This change in stiffness must be taken into account in changing the natural frequency of the system or investigating the possible occurrence of the resonance phenomenon.

Experimental and numerical studies on Steel Plate Shear Wall (SPSW) and its successful performance under last earthquakes have introduced this system as a lateral bearing system. There are lots of unknown information about SPSW despite of numerous studies that have were done. The effect of crack on the SPSW behavior is one of unknowns about SPSW that due to its complicity in nonlinear analysis, it has not been evaluated comprehensively. Because of thin steel plate and inherent of welding, emerging of crack on SPSW is deniable. So, in this paper the effect of central and edge crack and their growth on the behavior of SPSW have been studied. Results showed that the central crack are more destructive than edge cracks. The crack with long length lead the SPSW to fracture in elastic zone. Due to difficulty of crack modeling and crack analysis, the necessary relations have been proposed to obtain pushover diagram that without FE modeling the proposed relation estimate the pushover diagram of system in good agreement with FE results.

Passive energy dampers have been identified as efficient and economical tools to improve the seismic behavior of structures against seismic loads. Among all types of passive energy dampers, steel dampers are more popular because of their ease of construction, availability, affordability, economical aspect. In addition, these types of dampers have performed well in the laboratory and numerical studies, as well as in past earthquakes. Although these types of dampers are more economical than other dampers, they are not economical to use in conventional structures compared to other conventional lateral load systems. Therefore, in this paper, an innovative damper with shear mechanism is introduced which is easy to construct and operate and is economical to use. This proposed damper is easily replaceable after severe earthquakes. Numerical results show that the proposed damper improves the seismic behavior of the RC frame. It is usually not easy to increase the stiffness and ductility of the structure at the same time, but numerical results show that the proposed dampers increase the stiffness and ductility of structures. It also causes energy dissipation to the structure by increasing the damping in the nonlinear area. In this paper, the necessary equations for calculating the ultimate stiffness and strength of the system as well as its design are proposed.

In the steel plate shear wall, the thin steel plate is connected to the boundary elements, Therefore, forces transfer from the panel to the columns and increase the required cross sectional area of the columns. Separating the steel panel from the columns, reduces the seismic demands on the columns and prevent from the premature failure of the columns and the collapse of the entire structure. However, this technic decreases the lateral resistance of the system, therefore in such a case, using high yield steel (HYS) for the panel or utilizing a thicker steel plate may be required. In this paper, the seismic behavior of the steel plate shear walls connected to frame beams only (SSW-BO) is studied through nonlinear finite element analysis by ABAQUS software. After validating the numerical model against available experimental results, the effect of the edge stiffeners on the behavior of SSW-BO is explored. Based on the results, changing the edge stiffener cross section does not influence the seismic behavior, the stiffness, the lateral resistance and the dissipated energy of SSW-BO if the stiffeners are not connected to the top and bottom beams. However, connecting the stiffeners to the beams, drastically improve the seismic behavior and hysteretic characteristics of SSW-BO. It should be noted that, in such a case, greater forces may apply to the beams which should be considered in the seismic design of these members.

Y-shaped steel braces one of the systems is the lateral load. These braces, despite having the performance good architecture and resistance and hardness acceptable, they are not ductility. The use of dampers, such a rotary friction damper can increase their ductility capacity. The purpose of this paper is to evaluate the seismicity of y-shaped braces with rotary frictional damper. For this purpose, a frame of a floor and a span with y-shaped brace was selected and evaluated in two cases with and without rotary frictional damper. In evaluating these two frame, the stiffness, strength and energy absorption were considered. The results show that the strength and stiffness in the case with the damper decreases by 20% and 35%, but the energy absorption increases by 50%.

In the conventional steel plate shear walls (SPSWs), the steel plate is connected to all boundary elements, but in the steel plate shear walls connected to frame beams only (SSW-BOs) the plate is not connected to columns. The separation of the plate from the columns decreases the flexural demands of the columns and prevents the premature failure of the columns. In this paper, the effect of opening properties on the seismic behavior of SSW-BOs is studied through nonlinear finite element modeling and analysis. Based on the obtained results, initial stiffness, lateral strength and dissipated energy were higher in the specimen containing square opening compared to the one with circular opening. Moreover, increasing the opening dimension altered the tension field path from a line along the diagonal of the panel to the corners of the opening and degraded the seismic performance of SSW-BO. The most critical opening location is the central one since it disturbs the lateral load path in the panel more than any other locations. Finally, the results of numerical simulations are used to propose a simple method for estimation of the lateral stiffness and strength of SSW-BOs with openings based on the stiffness and strength reduction factors and the corresponding values of the solid SSW-BOs.